We need to pay off a medical bill before it goes to col

I am in a bit of a bind right now. I have debts up to my ears. I can handle everything with my job, but a medical bill for my wife keeps threatening to go into collections. She slipped while walking on a trail last year and fractured her ankle. We didn't have health insurance, the park rangers forced her to take an ambulance. I was able to negotiate the ER bill down, but the ambulance company charged $1400 for a 2 mile drive to the hospital! I have been paying it down but they keep demanding more money and I do not want this on her credit record. Please help.